The closed-form analytic solutions for the breakdown voltage of 6H-SiC RTD(silicon carbide reachthrough diode) having metal$-n^--n^+$ Schottky structure or $p^+-n^--n^+$, are successfully derived by solving impact ionization integral using an effective ionization coefficient. For the lightly doped n- epitaxial layer, the breakdown voltage of SiC RTD are nearly constant with the increased doping concentration while the breakdown voltages decrease for the heavily doped epitaxial layer.